1. Field of the Invention
The present invention relates to a technique for inspecting the surface state of a predetermined inspection object, for example, a curved face of a substance having a curved surface such as a soldered portion formed on a component mounting substrate, the inclination state of a component lead or the like.
2. Description of the Background Art
The applicant previously developed an apparatus for automatically inspecting a soldered portion on a substrate by an image processing method by utilizing the mirror reflecting property of the soldered portion (see the Patent Document 1).
[Patent Document 1]
Japanese Patent Publication No. 6-1173 (1994/1173)
FIG. 8 shows the structure of a substrate inspecting apparatus and the principle of an inspection which have been disclosed in the Patent Document 1. The inspecting apparatus serves to generate an image of an inspection object by three light sources 8, 9 and 10 for emitting lights having colors of red (R), green (G) and blue (B) and an image pick-up device 3, and the light sources 8, 9 and 10 are set such that an elevation angle formed with respect to a substrate surface is increased in order of R, G and B. On the other hand, the image pick-up device 3 is provided to pick up the image of a solder 2 to be an inspection object in a position placed just above.
The lights irradiated from the light sources 8, 9 and 10 are mirror reflected by the surface of the solder 2. In the optional position of the solder 2, a light emitted in a direction which is symmetrical with respect to the direction of the image pick-up device 3 seen from the same position is mirror reflected and is thereby guided to the image pick-up device 3. According to the optical system, therefore, a two-dimensional image having the colors R, G and B divided by the inclination of the solder surface is generated as shown in FIG. 9. In a spherical solder shown in the example, a flat surface in a central portion appears as a red image region, a steep surface in the vicinity of the substrate surface appears as a blue image region, and furthermore, a comparatively gentle inclined surface (moderate inclined surface) appears as a green image region. Based on the distribution state of the colors corresponding to the light sources, it is possible to decide the quality of the shape of the solder 2.
The inspection based on the principle described above is not restricted to a spherical solder but can also be applied to the inspection of the shape of a fillet. FIG. 10 shows the distribution state of the colors in the case in which the fillet on the substrate is observed by the optical system corresponding to the inclination state of the fillet. In FIG. 10, 50 denotes a substrate surface and S1 denotes a formation range of a land.
Also in the example of FIG. 10, the reflected light which is observed is divided, by the inclination angle of a solder, into blue on an upper steep surface, green on a middle steep surface and red on a surface which is almost flat in the vicinity of the substrate surface.
In the inspecting apparatus, a predetermined binary threshold is preset to each of the colors R, G and B. The image obtained by the image pick-up device 3 is changed into a binary value by the binary thresholds to extract a binary pattern for each of R, G and B (which will be hereinafter referred to as a “color pattern”). Referring to the color pattern, moreover, a pattern obtained in the image of a solder having an excellent shape is previously registered and the feature of each color pattern on the image of an inspection object is compared with that of the registered pattern, thereby deciding the quality of the surface state of the solder.
In a recent component mounting substrate, the size of a land is reduced with an increase in a density. In a small-sized substrate such as a substrate for a mobile telephone, particularly, the size of the land is to be reduced. However, when the size of the land is thus reduced, the inclination of the fillet is rapidly increased so that it is hard to extract each of the color patterns of R, G and B.
FIG. 11 shows an example of observation in the case in which the inclination of the fillet is very great. In this example, a formation range S2 of the land is reduced. Consequently, the fillet is inclined at an angle corresponding to the light source 10 for blue over a whole length so that most of a reflected light which is observed might be blue in some cases.
The same problem arises in the case in which the inclination of a component lead is to be observed. For example, in the case in which the upper surface of the lead is to be observed by using the optical system in FIG. 8, a very small change in an inclination angle which is caused by a float generated on the lead is included in a detection range of a normal red light. For this reason, it is hard to detect the very small float of the lead.
In the case in which the inclination state of an object having a very great inclination is to be observed in detail or the very small change of the inclination angle is to be detected, thus, it is hard to take measures in the optical system.
On the other hand, an increase in the number of the light sources can be proposed as a method of enhancing a resolution related to the detection of an inclined surface. In this case, a cost is increased.